1. Field of the Invention
This invention relates to a multipath laser apparatus using a solid-state slab laser rod that is able to implement a solid-state laser apparatus with a high efficiency in the conversion of excitation light to laser light, and can be applied to Nd:YAG laser apparatus, Nd:YLF laser apparatus and Yb:YAG laser apparatus, for example.
2. Description of the Prior Art
The efficiency of conversion of electrical input to laser output in a solid-state laser apparatus is the most important among various factors affecting the performance of a laser. In lamp-excited lasers, the efficiency of the conversion of electricity to light by the lamp itself is poor and its light-emission spectrum is wide, so much of the light cannot be absorbed by the laser rod, so the laser apparatus must have a large power consumption. In laser diode (LD)-excited lasers, the conversion efficiency of the LD itself is high and the spectrum of the emitted light is tuned to the absorption peak of the rod, so a higher conversion efficiency is obtained, but this is still not enough.
The causes of this include the small excitation light absorption cross section of the active ions of the solid-state laser that prevents the laser rod from efficiently absorbing all of the excitation light, and the small induced emission cross section that prevents the energy stored in the rod from being efficiently converted to laser light.
In order to increase the conversion efficiency of a laser apparatus, these properties must be compensated for by some method. Previous known methods of increasing the energy conversion efficiency of a laser apparatus include methods (1) and (2) below.
(1) To Increase the Doping of Laser-Active Ions in the Solid-State Laser Rod.
This is a method of increasing the active ion concentration to increase the excitation light absorption efficiency and laser gain. For example, U.S. Pat. No. 3,640,887 Latent Reference 1) and U.S. Pat. No. 3,897,358 (Patent Reference 2) recite the characteristics and method of manufacture of a ceramic laser rod containing neodymium (Nd) and other rare-earth ions. With a neodymium YAG (Nd:YAG) laser, the doping of laser-active ions can be increased in the ceramic more than in the single crystal. Moreover, larger-sized laser rods can be easily manufactured. In addition, “Optical properties and laser characteristics of highly Nd3+-doped Y3Al5O12 ceramics,” Applied Physics Letters, Vol. 77, No. 7, pp. 939-941 (2000) (Non-Patent Reference 1) and “72W Nd:Y3Al5O12 ceramic laser,” Applied Physics Letters, Vol. 78, No. 23, pp. 3586-3588 (2001) (Non-Patent Reference 2) also describe improvement of the oscillation characteristics of Nd:YAG ceramic lasers.
However, if the doping of laser-active ions is increased, disturbances in the refractive index distribution of the laser rod typically become evident. In addition, drops in optical quality such as reduced high-level lifetime in excitation occur more readily. These manifestations of disturbances in the refractive index distribution and drops in optical quality may become the cause of deterioration in the transverse modes of the laser beam thus obtained, and bring about performance drops as a laser medium. There are thus limits how much the doping can be increased.
In addition, in fundamentally transverse mode-oscillating lasers and high-power lasers, if there is heating within the interior of the laser rod, then this generates temperature distributions in the interior, thus giving the laser medium distortion. This distortion gives rise to decreased quality in the laser rod, birefringence and the like, so the transverse modes of oscillation diverge from the fundamental transverse mode. In order to prevent this, it is necessary to decrease the doping of active ions in the laser rod.
In addition, in order to increase the conversion efficiency to laser light, it is possible to use a laser rod with decreased doping to the level that high optical quality is kept, but in order to do this, it is necessary to:
(2) Use a Laser Rod that has a High Absorption of Excitation Light and has a High Laser Gain.
In order to increase the absorption of excitation light, the propagation length of excitation light in the rod must be made sufficiently longer than the absorption length (the length before attenuating to l/e, where e is the base of the natural logarithm). In other words, it is necessary to increase the excitation volume.
On the other hand, in order to increase the laser gain, it is necessary to raise the excitation density of the laser rod and thus increase the laser gain and also decrease the diameter of the oscillation mode and make the fundamental transverse mode more readily selected, or namely reduce the mode volume.
Thus, it is necessary to increase the excitation volume and reduce the mode volume in order to increase the energy conversion efficiency of the laser apparatus. However, these are contradictory conditions so reconciling them is difficult in a typical laser apparatus according to the prior art.
However reconciling these conditions is known to be possible by adopting multiple optical paths within the laser rod. Here, adopting multiple paths means causing the laser light to be reflected back and forth within the laser rod over optical paths that are shifted spatially a little at a time. It is thus possible to increase the excitation volume and increase the excitation light absorbance. Moreover, this causes the refractive index to vary uniformly within the laser rod, so the laser beam cross section is resistant to widening and the mode volume is kept small. In addition, in order to effectively use the energy stored within the laser rod, it becomes necessary to make the laser beam cross section equal over all sections of the laser rod, thus reducing the optical strength for induced emission.
As a result, the optical path length within the laser rod is multiplied by the rod length multiple, so the laser gain can be increased and finally the saturation amplification gain can be obtained in the laser rod used as the laser amplifier. Obtaining the saturation amplification gain in this manner is one indicator that the accumulated energy can be effectively converted to laser light, so many methods of adopting multiple paths have been proposed.
For example, U.S. Pat. No. 4,902,127 (Patent Reference 3) shown in FIG. 1 discloses a triple-path solid-state laser amplifier for eye-safe wavelengths comprising a slab rod and mirror reflectors. In addition, U.S. Pat. No. 5,172,263 (Patent Reference 4) discloses a quadruple-path solid-state laser amplifier comprising an orthogonal prism and plane mirrors. In addition, U.S. Pat. No. 5,615,043 (Patent Reference 5) discloses a confocal resonator that can be adapted to any of a solid-state laser, a liquid laser and a gas laser, along with a multipath laser amplifier that uses white cells. Moreover, U.S. Pat. No. 5,751,472 (Patent Reference 6) discloses multiple optical paths of a parametric oscillator/amplifier based on a 180° return prism and lenses and mirror reflectors. In addition, JP H10-041565 A discloses an octuple-path laser amplifier based on a wavelength plate, optical rotator and hexagonal laser rod.
In addition, as shown in FIG. 2, recently “Diode-pumped High-power CW and Modelocked Nd:YLF Lasers,” Advanced Solid-State Lasers, 2000 TOPS Vol. 34, Optical Society of America, Washington D.C. (2000) (Non-Patent Reference 3) describes an Nd:YLF laser wherein an increase in the overall efficiency and oscillation in transverse fundamental modes are obtained by a method wherein plane mirrors are placed at either end of a laser rod, and the light is caused to be reflected back and forth several times within the rod. In each of these methods, the multiple paths are implemented using mirror reflectors, reflector prisms or other reflectors outside the laser rod.
However, the number of optical components required to provide multiple optical paths in these laser apparatus becomes large. Moreover, precise adjustment is required in their placement. They thus have drawbacks in that the structure of the laser apparatus becomes complex and the size of the apparatus itself becomes large.
Moreover, because optical losses are unavoidable in the non-reflective coating at the laser rod ends and in the reflection by reflectors, these losses accumulate each time the laser light travels back and forth between the ends of the laser rod, resulting in the effect of the increased laser gain being reduced. Moreover, with the conventional multipath devices, adjacent optical paths vary in direction by only a very small amount, so they had a drawback in that the path distribution within the rod could not be adequately averaged.
An object of the present invention is to provide a laser rod with multiple paths that overcomes these drawbacks to give a high conversion efficiency and is able to readily generate high-quality laser light with regard to the oscillation mode.
In this manner, multiple optical paths are used in laser apparatus in order to increase the energy conversion efficiency of the laser apparatus, but the number of optical components required to provide multiple optical paths in these laser apparatus becomes large, and moreover precise adjustment is required in their placement, so there are drawbacks in that the structure of the laser apparatus becomes complex and the size of the apparatus itself becomes large. In addition, with nonlinear optical elements, the relationship between the direction of propagation of the excitation light and generated light within crystals is normally determined as a relationship that satisfies the phase matching conditions with respect to the crystal axes. For this reason, with nonlinear optical elements, they are fundamentally manufactured so as to have a straight, one-pass optical path or a single back-and-forth optical path, and thus they do not have multiple optical paths and do not give adequate wavelength conversion efficiency.
With this invention, it is possible to obtain a solid-state laser apparatus with a high efficiency of conversion from excitation light to laser light.